Pressure-sensitive adhesive



Patented May 8, 1951 PRESSURE-SENSITIVE ADHESIVE William E. Holland,Plainfield, N. J., Charles Olson Pike, Sanford, N. 0., and Emanuel S.Voutetakis, Paris, France, assignors to Industrial Tape Corporation, acorporation of New Jersey No Drawing. Original application November 22,

1944, Serial No. 564,730. Divided and this application November 10,1948, Serial No. 59,412

7 Claims.

v This invention relates to pressure-sensitive adhesives and to a methodof making the same. More particularly, it relates to an adhesivesuitable for use on pressure-sensitive adhesive tapes which aresubjected to a wide variety of climatic conditions and to the hightemperatures encountered in numerous industrial operations.

Adhesive masses employed in a multitude of hitherto knownpressure-sensitive adhesive tapes use rubber or a similar highltemperature-sensitive elastomer as the cohesive constituent of theadhesive mass. Due to the temperature-sensitivity of these cohesiveconstituents, adhesive masses made therefrom show extreme softness athigh temperatures and brittleness at low temperatures. Rubber adhesives,for instance, become soft at 80 to 100 0. They lose their cohesivestrength at such temperatures with the result that they separate fromthe tape backing and flow beyond the extremities thereof, causingsmudging of the underlying surface. This makes it necessary to removethe deposited material from the surface to which the tape has beenapplied in a special cleaning operation after removal of the tapeitself.

At lower than normal temperatures, on the other hand, adjacent layers ofadhesive sheeting, whether in the form of rolls or stacks, and whenconventional rubber base adhesives are ernployed, will freeze togethermaking it almost impossible to separate them for use. At still lowertemperatures, rubber base adhesives lose their adhesive properties,rendering entirely useless the adhesive sheeting to which they areapplied.

In accordance with the present invention there is provided an improvedpressure-sensitive adhesive which uses as the adhesive constituent ofthe mass a vulcanized copolymer of an aliphatic isomono-olefin and arelativel minor amount of a diolefin, sufficient to give the copolymerunsaturation corresponding to an iodine number of from 1 to 2 per centof the characteristic iodine number of natural rubber. This low amountof unsaturation is sufficient to allow vulcanization of the copolymer.The copolymers have a molecular weight of between 40,000 and 80,000 andare commonly known as butyl rubbers. Although satisfactory adhesives canbe made from all butyl rubbers, those most likely to be used arecopolymers of isobutylene and from 2 to 15% of a diolefin having aconjugated system of alternate single and double bonds, e. g., abutadiene includin substitute butadienes such as isoprene, since suchcopolymers are commercially available in this and other countries.

Perhaps it should be pointed out that the improved elastomer isdifferent from a mere mixture of a polyisomono-olefin and polymerizeddiolefin.

The isobutylene in a mixture of the latter type cannot be cured whereasbutyl rubber may be cured because of the residual unsaturation in thediolefin portion of the copolymer.

The improved pressure sensitive adhesive has excellent aging qualities,It ages better than adhesives made from polyisobutylene itself, which,according to the patent literature, have an aging life of two to tentimes greater than adhesives made from natural rubber. The improvedadhesive gives satisfactory service even after long storage. It has verylittle temperature sensitivity and can therefore be used with equaladvantage in tapes designed for very high or very low temperatureservice. It will suffer substantially no deterioration in quality attemperatures as high as C. and at least as low as 29 C. This improvedadhesive is thus suitable for use in the aircraft, automotive,typewriter, and other machine industries where pressure-sensitivemasking tapes have to withstand high temperature treatment of bakedfinishes and enamels. It may also be used in similar masking anddecorating operations involved in the finishing of elec-,

., tance in connection with aircraft designed for high altitude flyingand in packaging food and other perishables for long storage at lowtemperatures in the cold locker industry. Additional advantages are highresistance to oxidation and sunlight and impermeability to gases, thelatter property being an important characteristic of the improvedvulcanized butyl rubber adhesives. The adhesives are thus suitable forprolonged outdoor use and for application where a tight seal against gaspenetration is important.

An adhesive made in accordance with the invention may include 24 to 76%butyl rubber, 10 to 46% of tackifying agent, up to 50% of filler ofwhich at least 2% should be zinc oxide and ouring agents inamountsvvarying from 0.5 to 5% based upon the amount of the butylrubber, the percentages given being by weight. An optional addition ofup to 35% of a plasticizer will soften the mass and improve itsspreading and adhesive qualities.

Any compatible resin tackifier will serve as a tackifying agent. Suchresin tackifiers are well known in the art and include, by way ofexample only, rosin or, its'derivatives, polymerized terpene resins,derivatives of pentaerythritol, coumaroneindene resins, petroleumresins, dammar resins, etc.

The preferred filler is zinc oxide although others may be used, such astitanium dioxide, iron oxide, hydrated alumina orclay, provided theabove mentioned minimum percentage of zinc oxide is also present.Transparent adhesives may be made according to the present inventionusing the minimum amount of zinc oxide, or without any filler whatsoeverin those cases where the cures do not rely upon sulfur or sulfurbearingcuring agents.

Among the suitable plasticizers that may be used are mineral oil,commercial petroleum hydrocarbon oils such as Circo Light Oil, a productof the Sun Oil Company, which is largely alicyclic and consistsprincipally of hydrocarbons containing 25 to 30 carbon atoms permolecule and one or more naphthenic or aromatic rings in the molecule,the non-volatile solvent oil sold by the Neville Company under thetrade-mark Nevinol" and which has a distillation range between 300 and370 C., and the clear mobile hydrocarbon oil sold commercially by theMonsanto Chemical Company under the designation HB- 40 and which has adistillation range between 345 and 450 (3.; the liquid chlorinateddiphenyls; and a liquid coumarone-indene polymer sold by the MonsantoChemical Company under the trade-mark Tackol. A variety of other equallysuitable plasticizers which also may be used will readil occur to anyoneskilled in the art.

The amounts and types of curing agents may vary depending upon the exactconsistency of the adhesive mixture and upon the resin used. Among thepreferred curing agents are: quinoids (e. g. p-quinone dioxime) whichare preferably used in combination with peroxides; organic or in,-organic peroxides (e. g. benzoyl peroxide, t-butyl hydroperoxide,t-butyl peroxide, acetyl peroxide, lead dioxide, calcium peroxide orbarium peroxe ide); thiazoles (e. g. 2 mercapto benzothiazole); thiurainsulfides (e. g. tetramethyl or tetraethyl thiuram disulfide,di-N-pentamethylene thiuram tetrasulfide) and the dithiocarbamates (e.g.

amine salts such as n-pentamethylene ammoni-. um pentamethylenedithiocarbamate, zinc salts.

such as zinc dimethyl dithiocarbamate, selenium salts such as seleniumdiethyl dithiocarbamate or complex mixtures of dithiocarbamatessuch asthe one sold by the R. T. Vanderbilt Co. as Butyl Eight). Aldehyde-aminecondensation products or guanidines may also be used, but these havebeen found to be relatively slow in connecticn with butyl rubbervulcanization.

The foregoing data will enable one skilled in the art to choose a curewhich will properly vulcanize a butyl rubber adhesive in from A; to 3hours. The cures are simple and easily controlled due to the extremelyslow rate of vulcanization which is characteristic of butyl rubber. Aseries of cures which have proven particularly reliable are listed belowby way of example only. The amounts given are percentages based onthebutyl rubber or equivalent polyisomonoolefln-diolefin copolymer contentof the compound:

Example of Cures Curing Agent p-quinone dioxime tbutyl hydropcroxideLead dioxide Z-mercapto benzothiazol Di-n-pentamethylcne thiururntetrasulfide Zinc dibutyl (lithi0curbamflte Selenium diethyldithiocorbamate 1.00 2.00

Sulfur Approximate curing speed at 100 C. during constant mastication,hours 0.3-1. 5 l 2. 5

In compounding the improved adhesive mass, it is preferred toincorporate the filler and the curing agent completely into the butylrubber on a cold mill. The resin is then added and the composition mixedin a cold masticator for one hour, after which the temperature israised, preferably above C. and held for an appropriate period of timeto effect a cure. Alternatively the butyl rubber compound including thefiller and curing agents may be cured without mastication andthen milleddown to spreading consistency. The conditions of cure should be adjustedforminimum plasticity in keeping with the ability of the cured copolymerto form a solution of at least 20 per cent solids in straight aliphaticsolvents such as normal heptane. The improved adhesive may be spreadupon abacking material to produce a pressure sensitive adhesive tapefrom solutions of that. character. In general, lower solubilities arenecessary where hot spreading issubstituted for solvent spreadingalthough the latter method is preferred.

The presence of at least a small amount of zinc oxide is necessary inall of the above mentioned examples of cures, except Examples A and Bwhich do not rely on sulfur for vulcanization. The preferred range of'constituents for vulcanized butyl rubber adhesives has been stated. Thefollowing specific examples falling within the given range areillustrative of the various compounding procedures that may be used. Allproportions are by weight:

Example] Per cent Butyl rubber 37.5 Zinx oxide i 37.5 Nevillite #12312.5 Circo Light Oil 1.2.5

Example 2 Per cent Butyl rubber 30 Zinc oxide 30 Glycerol ester ofhydrogenated rosin H 20 Mineral oil 20 Example 3 Per cent Butyl rubber30 Zinc oxide 10 Titanium dioxide 17 Bolypale Resin 23 Circo Light Oil20 Example 4 Per cent Butyl rubber 31 Zinc oxide 27 Glycerol ester ofhydrogenated rosin 25 Circo Light Oil 1'7 Example 5 Per cent Butylrubber 49 Glycerol ester of hydrogenated rosin 25 Circo Light Oil 26 Itwill be understood that each of the formulas given in Examples 1 to 5requires a cure which may be selected from the proup of cures A to Epreviously referred to, although any suitable cure will sufiice. If cureA is used with the formula given in Example 5, a butyl rubber adhesivehaving inherent transparency will result and which, therefore, may beused for tapes having transparent backings. Substantially transparentadhesives may also be made using, for instance, cures C, D, and E,provided the butyl rubber formula contains only a small amount of zincoxide.

As a further identification of certain of the constituents mentioned inthe examples, it should perhaps be stated that Nevillite #123 is acoumarone-indene resin, sometimes referred to as a cycloparaffin andhaving a melting point approximately 115 C. It is manufactured by theNeville Co. A suitable glycerol ester of hydrogenated rosin is sold byHercules Powder Co., under the name Staybelite Ester #10. Polypale Resinis a rosin derivative, regarded as a dipolymer of abietic acid and islike wise sold by Hercules Powder Co. Circo Light Oil has beenidentified heretofore.

The invention has been described with reference to its preferredembodiments although many modifications thereof are included within itsspirit. The invention, therefore, is limited only by prior art and thescope of the appended claims.

This application is a division of application Serial No. 564,730, filedby the same inventors on November 22, 1944, and now abandoned.

The claims are:

1. For an adhesive tape, a pressure-sensitive adhesive mass consistingof a cohesive constituent including a cured copolymer of isobutylene andfrom about two to about fifteen per cent by weight of a butadiene, saidmass also consisting of, on a weight basis, from about ten to aboutforty-six per cent compatible resin up to about thirty five per cent ofcompatible plasticizer, and up to about fifty per cent filler, saidcopolymer being cured to the range providing substantially minimumplasticity consistent with a solubility of the order of about twenty percent in straight aliphatic solvent of the normal heptane type.

2. For an adhesive tape, a pressure-sensitive adhesive mass according toclaim 1 in which the filler contains zinc oxide to the extent of notless than about two per cent by weight of the adhesive mass.

3. For an adhesive tape, a pressure-sensitive adhesive mass according toclaim 1, wherein the copolymer has a molecular weight between about40,000 and about 80,000.

4. In an adhesive tape, a pressure-sensitive transparent adhesive massconsisting of a cohesive constituent including a cured initiallyslightly unsaturated elastomeric copolymer of isobutylene and dioleln,which has a degree of vulcanization in the range providing substantiallyminimum plasticity consistent with a solubility of the order of about 20per cent in straight aliphatic solvent of the normal heptane type, andfrom about ten to from about forty-six per cent compatible tackifier.

5. For an adhesive tape, a pressure-sensitive adhesive mass according toclaim 4, wherein the copolymer has a molecular weight between about40,000 and about 80,000.

6. In an adhesive tape, a pressure-sensitive transparent adhesive massconsisting of a cohesive constituent including a cured, initiallyslightly unsaturated, elastomeric copolymer of isobutylene and an amountof diolefin providing unsaturation of the copolymer corresponding to aniodine number of from about one to about 2 per cent of thecharacteristic iodine number of natural rubber, which cohesiveconstituent has a degree of vulcanization in the range providingsubstantially minimum plasticity consistent with a solubility of theorder of about 20 per cent in straight aliphatic solvent of the normalheptane type, from about 10 to about 46 per cent by weight of theadhesive mass of compatible tackifier, and a minor portion of liquidplasticizer comprising not more than about 35 per cent.

7. For an adhesive tape, a pressure-sensitive adhesive mass according toclaim 6, wherein the copolymer has a molecular weight between about40,000 and about 80,000.

WILLIAM E. HOLLAND. CHARLES OLSON PIKE. EMANUEL S. VOUTETAIC[S.

No references cited.

1. FOR AN ADHESIVE TAPE, A PRESSURE-SENSITIVE ADHESIVE MASS CONSISTINGOF A COHESIVE CONSTITUENT INCLUDING A CURED COPOLYMER OF ISOBUTYLENE ANDFROM ABOUT TWO TO ABOUT FIFTEEN PER CENT BY WEIGHT OF A BUTADIENE, SAIDMASS ALSO CONSISTING OF, ON A WEIGHT BASIS, FROM ABOUT TEN TO ABOUTFORTY-SIX PER CENT COMPATIBLE RESIN UP TO ABOUT THIRTY FIVE PER CENT OFCOMPATIBLE PLASTICIZER, AND UP TO ABOUT FIFTY PER CENT FILLER, SAIDCOPOLYMER BEING CURED TO THE RANGE PROVIDING SUBSTANTIALLY MINIMUMPLASTICITY CONSISTENT WITH A SOLUBILITY OF THE ORDER OF ABOUT TWENTY PERCENT IN STRAIGHT ALIPHATIC SOLVENT OF THE NORMAL HEPTANE TYPE.